(a) Field of the Invention
The present invention relates to a cathode active material for a secondary battery and, more particularly, to a cathode active material for a secondary battery, which includes a spinel-structure lithium manganese composite oxide exhibiting a 5-volt-class operational potential and having a large discharge capacity.
(b) Description of the Related Art
Lithium-ion secondary batteries are widely used for portable data-processing terminals such as personal computers and mobile telephones. There has been a technical subject such that the secondary batteries should have smaller dimensions and a lower weight, and the current important technique subject is that the secondary batteries should have a higher energy density.
There are some conceivable techniques for increasing the energy density of the lithium-ion secondary battery. Among other techniques, it is considered highly effective to raise the operational potential of the lithium-ion secondary battery. In the conventional lithium-ion secondary batteries using lithium cobalt oxide or lithium manganese oxide as a cathode active material, the operational potential of the cathode against a lithium reference electrode is limited to a 4-volt class, i.e., around 4 volts or between 3.6 and 3.8 volts at the average operational potential. This limit of the operational potential results from the fact that the appeared potential is limited by the oxidation and reduction reactions of cobalt ions or manganese ions such as “Co3+Co4+” or “Mn3+Mn4+”.
On the other hand, it is known that a spinel compound wherein Mn in the lithium manganese oxide is substituted by Ni etc., if used as the active material, can achieve an operational potential of 5-volt class, i.e., as high as around 5 volts. More specifically, use of the spinel compound such as LiNi0.5Mn1.5O4 as the cathode active material provides a potential plateau in the range above 4.5V. In such a spinel compound, Mn exists in the form of tetra-valence, wherein operational potential is defined by the oxidation and reduction reactions of Ni2+Ni4+ which replaces the oxidation and reduction reactions of Mn3+Mn4+.
However, even the energy density of the spinel compound of LiNi0.5Mn1.5O4 etc. does not significantly exceed the energy density of LiCoO2 heretofore, and accordingly, a substance for the active material having a further higher energy density and a further higher storage capacity has been desired.
In addition, the spinel compound such as LiNi0.5Mn1.5O4 suffers from the problems such as reduction in the discharge capacity after iterative charge and discharge cycles and degradation of the crystal structure at a higher temperature range, and these problems should also be removed.
It is noted that the technique of replacing manganese and oxygen by other metals has been often used in the 4-volt-class active materials. For example, Patent Publications JP-A-11-312522 and -2001-48547, some of manganese in lithium manganese oxide is substituted by nickel while introducing metals such as boron for improving the cycle characteristics and preservability of the battery at a higher temperature. The purpose of the substitution in the present invention, however, differs from the purpose of the substitution in the 4-volt-class active material.
In JP-A-2001-48547, the substitution of some of Mn by another element is conducted for the purpose of suppressing the reduction of the storage capacity due to the crystal distortion in the manganese oxide caused by iterative operation. It is recited in this publication that the amount of substitution should be maintained below a specified value for avoiding reduction of the storage capacity caused by the reduction of the tri-valent Mn. It is recited in JP-A-2001-48547 that, in the technique wherein some of Mn is substituted by lithium, some of the lithium is replaced by bi- or tri-valent other metals for suppressing the reduction of the tri-valent Mn to thereby prevent the reduction of the storage capacity. In particular, the valence of Mn is defined at 3.635 or lower in JP-A-11-312522. More specifically, the substitution of Mn in the conventional cathode active material of 4-volt class is effected while suppressing the valence of Mn at a lower value for maintaining the storage capacity. In these publications, in view that the operational potential of the active material is defined by the valence change of manganese, tri-valent manganese should remain at a specified amount in the active material, and thus the molecular ratio of nickel in the active material is in general 0.1 or below.